Conventional low bypass turbofan gas turbine engines include a bypass duct for bypassing a portion of fan air around the core engine thereof. The bypass duct is an annular channel defined between the engine outer casing and the inner casing spaced radially inwardly therefrom which surrounds the core engine. Combustion gases discharged from the core engine generate thrust for propelling an aircraft.
The aft end of the engine is typically mounted to the aircraft using a conventional mounting ring which is formed of a portion of the engine outer casing. The mounting ring also forms part of the outer surface of the bypass duct and is typically structurally connected to the turbine frame supporting the core engine. The turbine frame includes an outer casing which defines a portion of the inner surface of the bypass duct and also includes a plurality of circumferentially spaced struts extending radially inwardly therefrom between which combustion gases are discharged from the core engine. The struts are connected at their radially inner ends to an inner casing which supports a bearing for supporting rotor shafts of the core engine.
The mounting ring, and therefore the bypass duct, is conventionally connected to the turbine frame by using conventional turnbuckles. The turnbuckles are used to allow individual adjustment thereof for centering the turbine frame relative to the outer casing for allowing the bypass duct to be assembled concentrically with the mounting ring and the turbine frame. This centering procedure prolongs assembly time during manufacture of the engine and requires internal access to the turnbuckles in order to carry out the procedure. Furthermore, the turnbuckles are relatively large in cross section and provide obstruction to the flow of bypass air in the bypass duct, which therefore requires that the outer diameter of the outer casing be made larger to allow for additional flow area in the bypass duct to compensate for the flow area lost due to the presence of the turnbuckles.
Accordingly, not only do the turnbuckles themselves provide blockage of the airflow which results in aerodynamic pressure losses, but the bypass duct must be made larger to provide for a predetermined amount of flow area required through the bypass duct for a particular engine application.
Furthermore, in some engine designs, the rear mounting ring is not located in the plane of the turbine frame and therefore is not available for supporting the bypass duct to the turbine frame. Such support is preferred because thrust from the engine discharge gases provides reaction loads through the outer duct which tend to distort the outer duct into an oval shape, for example, which is undesirable, as well as induce vibratory exitation in the outer casing.